Since monomeric diisocyanates cannot be used as crosslinkers in polyurethane coating systems, owing to their volatility and toxicological properties, the general approach is to use the higher molecular mass derivatives which are low in monomers, examples being those on a uretdione, isocyanurate, biuret, urethane or allophanate basis. An overview of these polyisocyanates and how to prepare them is given for example in J. Prakt. Chem./Chem. Ztg. 1994, 336, 185–200.
The oligomerization of isocyanates by reacting typically two or three NCO functions with one another leads to the structures of the following formulae 1–3, the uretdione structure (type 1) and the isocyanurate structure (type 2) being the structures which are important industrially.
X=carbon skeleton
A multiplicity of covalent and ionic catalysts have been described in the literature as catalysts for this oligomerization (J. Prakt. Chem./Chem. Ztg. 1994, 336, 185–200). Uncharged compounds of covalent construction, however, exhibit a much lower activity than salt-like compounds, so that for a given conversion it is necessary either to use more catalyst or for the reaction time to be longer accordingly.
DE-A 3 100 263, EP-A 339 396 and EP-A 330 966 describe catalysts of salt-like construction, such as carboxylates, fluorides and hydroxides, for isocyanate oligomerization. These catalysts exhibit a high selectivity in respect of formation of isocyanurate (type 2), but hardly any dimer structure (type 1) is formed, or none at all.